Recently, electronic devices such as mobile phones, home appliances, PCs, PDAs, LCDs and navigators have been gradually digitalized and operated at high speed. Because such electronic devices are sensitive to a stimulus from the outside, their circuits may be damaged or signals may be distorted when small abnormal voltages and high-frequency noises are introduced into internal circuits of the electronic devices from the outside.
Switching voltages generated in circuits, power noises contained in power voltages, unnecessary electromagnetic signals, electromagnetic noises, and the like cause such abnormal voltages and noises. A filter is used as a means for preventing such abnormal voltages and high-frequency noises from being introduced into a circuit.
In a general differential signal transmission system, in addition to common mode noise filters for eliminating common mode noises, passive components such as diodes and varistors should be separately used to prevent ESD that may be generated at input/output terminals. If the separate passive components are used at the input/output terminals to cope with ESD, a mounting area is broadened, manufacturing cost is increased, and signal distortion or the like is generated.
For example, in order to prevent ESD using a varistor, one end of the varistor is connected to an input/output terminal, and the other end of the varistor is connected to a ground terminal, and thereby electronic components in an electronic device are protected. However, the varistor functions as a capacitor in a normal operating state of an electronic device to which a transient voltage is not applied. Since capacitance of a capacitor is changed at a high frequency, signal distortion or the like may be generated when a varistor is used at a high-frequency or high-speed data input/output terminal or the like.